**Core Concept**
Hirsutism is a clinical condition characterized by excessive growth of terminal hairs in a male pattern in women due to androgen excess. The underlying cause of hirsutism can be either hormonal or non-hormonal. Hormonal causes are often due to an imbalance of androgens such as testosterone and dihydrotestosterone (DHT).

**Why the Correct Answer is Right**
The most common cause of hirsutism is **Polycystic Ovary Syndrome (PCOS)**. PCOS is a complex endocrine disorder that affects 5-10% of women of reproductive age. It is characterized by hyperandrogenism, ovulatory dysfunction, and polycystic ovaries on ultrasound. The exact pathophysiology of PCOS is not fully understood, but it is thought to involve insulin resistance, abnormal gonadotropin secretion, and increased androgen production by the ovaries.

**Why Each Wrong Option is Incorrect**
* **Option A:** Congenital Adrenal Hyperplasia (CAH) is a rare genetic disorder that results in an excess of androgens, but it is not the most common cause of hirsutism.
* **Option B:** Hyperprolactinemia can cause hirsutism, but it is a less common cause.
* **Option C:** Thyroid disorders can cause changes in hair growth, but they are not a primary cause of hirsutism.

**Clinical Pearl / High-Yield Fact**
PCOS is often associated with other clinical features such as acne, male pattern baldness, and irregular menstrual cycles. A high index of suspicion is necessary to diagnose PCOS, as the symptoms can be subtle.

**Correct Answer: D. Polycystic Ovary Syndrome (PCOS)**